1. Field of the Invention
The present invention relates to an optical filter for correcting non-linear gain or attenuation in a wavelength band of the optical system by adding an insertion loss having a quadratic-shaped insertion loss curve in the wavelength band and having a maximum insertion loss at a center wavelength of the wavelength band.
2. Description of the Related Art
Optical fiber communications systems are advantageously and desirably used to transport data in the form of light pulses over long distances because they exhibit a very large capacity for carrying information, are light-weight, and are immune to electromagnetic interference. In some systems, several optical carriers of different wavelengths are used to simultaneously propagate the multiple wavelengths along the same fiber. These are referred to as Wavelength Division Multiplexed (WDM) systems and further increase the capacity of optical systems.
In a typical WDM system, optical fiber amplifiers are used to increase the power level of the optical signals and are typically arranged along a fiber path to keep the optical signal levels above the system noise. However, these optical amplifiers such, for example, as Erbium-doped fiber amplifiers (EDFA) typically exhibit wavelength dependent gains. The tendency is to have the gain at a maximum near the center of the wavelength band.
Furthermore, the attenuation rate of the transmitted signal in the optical fiber varies depending on the carrier wavelength, thereby causing some channels, i.e. wavelengths, to have a higher power level than others at the receiving end of the optical fiber. The difference in attenuation rates of different wavelengths within the fiber is known as tilt. To correct the effects of tilt--i.e. to flatten the gain over the wavelength band of the WDM system--power equalizers are introduced into the optical systems. One such power equalizer is described in commonly-owned U.S. patent application Ser. No. 09/217,710, filed Dec. 21, 1998, which discloses an active filter that compensates for gain tilt in WDM systems. However, it has been found that for overall wavelength ranges of 100 nm, this device still deviated by as much as 0.65 dB from linearity. This is because, similar to the optical amplifier, the effect of the power equalizer is not linear. Therefore, although the tilt effect is lessened, the equalized wavelength band is non-linear.
Accordingly, a problem in existing WDM optical systems, specifically those which include optical amplifiers and power equalizers, is that a non-linear effect exists across the wavelength band of the WDM system.